Removal of metal chalcogen scale



United States Patent 3,282,851 REMOVAL OF METAL CHALCOGEN SCALE Paul E. Muehlberg and Fred N. Teurnac, Lake Jackson,

Tex., assignor's to The Dow Chemical Company, Midland,Micl1., a corporation of Delaware No Drawing. Filed Nov. 5, 1962, Ser. No. 235,550 3 Claims. (Cl. 252152) The present invention relates to the removal of metal chalcogen scale and concerns aqueous solvent compositions which selectively remove metal chalcogen deposits from metal surfaces.

It is frequently desirable, and often essential, to clean metal surfaces which have acquired coatings or deposits of metal chalcogens. For example, the passage of large volumes of water in contact with various types of processing equipment frequently results in the deposition of small amounts of metal oxides on exposed metal surfaces of the equipment. This deposition of metal oxides becomes an especially serious problem in water processing equipment requiring close tolerance design such as, for example, steam turbines. Attempts to prevent deposition of metallic oxide, as by purification of influent water, have not satisfactorily solved this problem and, as an alternative solution, efforts have been directed towards methods for removing such deposits from metal surfaces.

One specific and economically important problem area is that of removing copper oxide deposits from fenruginous surfaces. A number of solvent compositions which have been previously employed for this purpose are objectionable because of indiscriminate attack on both copper oxide and elemental iron. Other solvent compositions which selectively dissolve copper oxide are unsatis factory when in contact with ferruginous surfaces because the resulting copper ions are preferentially displaced from solution by iron and elemental copper plates out on the metal surfaces.

Similar problems arise when the removal of other metal chalcogen scale from metal surfaces is necessary. For example, it is frequently desirable to remove sulfides and oxides of silver which have coated silver metal surfaces.

An effective and highly selective solvent composition has now been discovered, in accordance with the present invention, which dissolves metal chalcogen scale without attacking continguous metal surfaces and which maintains the dissolved scale in solution thereby preventing subsequent deposition of the so-dissolved metal species on contiguous metal surfaces. Examples of metal chalcogen scale which may be advantageously removed by use of the selective solvent composition of this invention include the oxides and sulfides of copper, silver, gold, iron, nickel and aluminum.

The solvent composition of the present invention com prises an aqueous mixture of (1) from about 5 to 40 weight percent polyethylenirnine (hereafter PEI) and (2) from about 2 to 20 weight percent of a hydroxylamine mineral acid salt. The mixture is employed in a PEIzhydroxylamine salt Weight ratio of greater than 1 with a preferred ratio being from about 2:1 to 5:1. At ratios of less than 1:1, normal variations in temperature tend to cause the metal ions in solution to form metal deposits. No benefit is incurred by the presence of PEI in excess of a 5:1 ratio.

The hydroxylamine salt employed is selected from the group consisting of hydroxylamine hydrochloride, hydroxylamine nitrate and hydroxylamine sulfate with the exception that the hydrochloride salt is not employed when metal scale is to be removed from equipment containing stainless steel which is attacked by hydroxylamine hydrochloride.

The PEI employed is prepared by polymerization of ethylenimine. Conventional methods, such as described by Jones et al., Journal of Organic Chemistry, vol. 9, pages 125-147 (1944), may be used to obtain the polymer. The molecular weight of the product is not critical although the difficulty of handling extremely viscous polymer solutions makes very high molecular weight PEI less desirable. The PEI utilized in the present invention may have a molecular weight as low as 500 although weight ranges from about 10,000 to about 150,000 are usually employed.

The temperature employed during the removal of metal scale, through use of the solvent compositions of this invention, may range from about 20 to 300 C. with a temperature range of from about 60 to C. preferred. When temperatures in excess of 200 C. are employed, some decomposition of the hydroxylamine salt may occur with the formation of corrosive products. Temperatures higher than 20 C. are preferred since the rate of metal chalcogen scale solution increases with increasing temperature.

The hydroxylamine salt and PEI are usually prepared in an aqueous solution which is then introduced into the equipment to 'be cleaned of metal chalcogen scale. Alternatively, a concentrate of the salt and PEI may be added to Water already within the equipment in proportions which will yield a solution of the desired concentration.

The following examples describe completely representative specific embodiments of the present invention. The examples, however, are not to be interpreted as limiting the invention other than as defined in the claims. Hydroxylamine sulfate and hydroxylamine chloride are designated by the letters HAS and HAC, respectively, in the tables.

Example 1.-Deposits of copper oxide were placed on steel coupons and the coupons immersed in aqueous solutions of PEI-hydroxylamine salt to determine the efiicacy and selectivity, of solutions of various concentrations, in dissolving copper oxide. After a 5 hour immension at 72 C. the steel coupon was removed and the solution was analyzed for copper and iron content. Table I, below, lists the results of analyses of a variety of solvent compositions employed in this procedure. The values are expressed as weight percentages of the aqueous solution.

Example 2.The following test procedure was employed to determine (1) the eflicacy of aqueous PEI: hydroxylamine salt solutions in dissolving copper oxide and (2) the ability of these solutions to prevent deposition of copper, after solution of the copper oxide, in the presence of iron.

A quantity of 100 grams of aqueous PEIzhydroxylamine salt solution was poured over an excess of copper oxide wire contained in a glass flask. A freshly pickled steel coupon was partially immersed in the flask contents. The flask Was equipped with a water-cooled condenser, to prevent evaporation, and was contained in a controlled temperature bath maintained at 72 C. After one hour and five hour intervals, the amount of copper oxide dissolved was determined and the steel coupon examined for copper deposition. The results are tabulated in Table II, below. The concentration of PEI and hydroxylamine salt are expressed as weight percent based on total aqueous solution weight. 1

TABLE II G. Copper Oxide Dissolved/ 100 g. Solution. N0. Composition 1 Hr. 5 Hr.

10% HAS plus 7% PEI 0. 62 2. 30 7% HAS plus 5% PEI.-." 0.35 1. 40 5% HAS plus 5% PEI 1. 10 2. 4% HAS plus 7% PEI 0. 90 l. 80 7% HAS plus 20% PEI 1. 46 3. 40 5% HAS plus 20% PEI 1. 40 3. 20 7% HAS plus 20% PEL... 1. 30 3. 20

1 10 g. of Fe powder added to solvent composition.

In Table II, above, a PEIzHAS ratio of 1:1 or less (Nos. 1, 2 and 3) resulted in some deposition of copper on the steel coupon. At ratios greater than 1:1 no copper deposition occurred. In No. 7 the addition of 10 g. of iron powder resulted in less than 10 p.p.m. of iron in the solution analyzed at the end of 4 hours. Replacement of hydroxylarnine sulfate -wtih hydroxylamine chloride results in a more rapid solution of copper oxide. Under the conditions of Example 2, a 20 weight percent PEI:7 weight percent hydroxylamine chloride solution 4 l will dissolve about 6 grams of copper oxide in 5 hours at a temperature of 72 C.

We claim:

1. A metal chalcogen scale removal solvent solution of (1) from about 2 to 20 Weight percent of a hydroxylamine mineral acid salt, (2) from about 5 to 40 weight percent of polyethylenimine having a molecular weight of at least 500, and (3) from about 40 to 93 weight percent water; said polyethylenimine and hydroxylamine salt being employed in a polyethylenimine:hydroxylamine salt weight ratio of from 1:1 to about 5:1.

2. The solution of claim 1 wherein the hydroxylamine mineral acid salt is selected from the group consisting of hydroxylamine sulfate, hydroxylamine hydrochloride and hydroxylamine nitrate.

3. The solution of claim 1 wherein the amount of hydroxylamine salt is from 5 to 10 weight percent and the amount of polyethylenimine is from 15 to weight percent with the remainder being water.

References Cited by the Examiner UNITED STATES PATENTS 2,995,512 8/1961 Weidner et al. 252-344 XR 3,003,970 10/1961 Call 252-152 3,042,641 7/1962 West et al. 26029.2

LEON D. ROSDOL, Primary Examiner.

JULIUS GREENWALD, Examiner.

W. E. SCHULZ, Assistant Examiner. 

1. A METAL CHALCOGEN SCALE REMOVAL SOLVENT SOLUTION OF (1) FROM ABOUT 2 TO 20 WEIGHT PERCENT OF A HYDROXYLAMINE MINERAL ACID SALT, (2) FROM ABOUT 5 TO 40 WEIGHT PERCENT O POLYETHYLENIMINE HAVING A MOLECULAR WEIGHT OF AT LEAST 500, AND (3) FROM ABOUT 40 TO 93 WEIGHT PERCENT WATER; SAID POLYETHYLENIMINE AND HYDROXYLAMINE SALT BEING EMPLOYED IN A POLYETHYLENIMINE-HYDROXYLAMINE SALT RATIO OF FROM 1:1 TO ABOUT 5:1. 